When a marine vessel is in motion under power, it is followed by a wake, and a particular point of the wake known as the "hot spot" is found to be radiometrically detectable more reliably than the vessel itself. When it is desired to attack such a vessel by proportional navigation of a ballistic missile launched from an aircraft or from a ground based launcher, it is known to equip the missile with an antenna system and a wake tracking computer such that the missile searches for and tracks the vessel's hot spot and impacts thereon.
The hot spot follows in the wake of the vessel at a distance behind the vessel which is related to the vessel's speed by a function which is essentially the same for practically all vessels, and which will be referred to here as the wake formation time constant. The distance in question is frequently great enough so that when a missile impacts on the hot spot, damage to the vessel itself is minimal.